1. Field Of The Invention
The present invention relates to surgical screws for fixation in bone and, more specifically, to bioabsorbable interference bone screws particularly useful in securing a ligament in a bone tunnel.
2. Description Of The Prior Art
Graft and prosthetic ligaments are utilized to surgically repair and/or replace ligaments damaged by injury or disease. Surgical procedures to repair and/or replace ligaments generally involve forming a tunnel in bone, positioning a graft or prosthetic ligament in the bone tunnel, and anchoring the ends. Various devices are typically employed to secure the bone blocks the ligament in the bone tunnel, including buttons, staples, expanding cones, unicortical screw posts, as well as interference screws. When interference screws are used, the screws are inserted into the bone tunnel to engage the tunnel wall and bone blocks at the ends of the ligament and, thus, provide an endosteum or endosteal fixation therebetween.
Surgical bone screws for fixation in bone and for anchoring ligaments to bone are typically fabricated from medically approved metallic materials that are not naturally absorbed by the body. An illustrative metallic bone screw is the M. Kurosaka.TM. bone screw manufactured by DePuy, a division of Boehringer Mannheim Corporation, and a further example of a metallic bone screw is shown in U.S. Pat. No. 4,754,749 to Tsou. Most metallic bone screws include a threaded shank joined to an enlarged head having a transverse slot or hexagonal socket formed therein to engage, respectively, a similarly configured, single blade or hexagonal rotatable driver for turning the screw into a bone. The enlarged heads on such screws can protrude from the bone and can cause chronic irritation and inflammation of surrounding body tissue. Metallic bone screws that do not have enlarged heads possess disadvantages because mismatch between screw length and the length of the ligament bone block can result in the screw being inserted too far, or not being inserted to its full length, in the bone tunnel. In anterior cruciate ligament repair and reconstruction, insertion of the screw too far can produce intraarticular penetration, and failure to insert the screw its full length can irritate adjacent soft tissue. Additionally, placement of screws in bone tunnels formed in movable joints can, in certain instances, cause abrading of ligaments during normal motion of the joint. Furthermore, bone screws occasionally back out after insertion; and, when this occurs, the bone screw can protrude into surrounding tissue and cause discomfort. Because metallic bone screws are not assimilated by the body, additional surgical procedures may be required to remove problematic bone screws once the fixated bone and/or tissue has healed.
Biodegradable bone screws have been proposed, as exemplified in U.S. Pat. No. 4,356,572 to Guillemin and International Application PCT/EP 89/00344, and as alluded to in U.S. Pat. No. 4,927,421 to Goble et al. Bioabsorbable bone screws possess the advantage of being naturally degradable by the body; and, therefore, contact with surrounding tissue after insertion does not necessitate surgical intervention because the screw will be completely absorbed by the body once the bone and/or tissue has healed. However, conventional bioabsorbable bone screws present numerous difficulties due to bioabsorbable materials being considerably softer than metallic compositions. In particular, bone screws made from bioabsorbable materials are susceptible to deformation and deflection when subjected to forces required to drive the screw into relatively hard tissue, such as bone, and the transverse slot and hexagonal socket typically provided in bone screws as drive recesses for receiving standard, similarly configured, rotatable drivers are unsuitable for bone screws fabricated of bioabsorbable material. The high torque that must be applied to bone screws by a driver to produce rotation of the screw in bone can cause shear deformation of the relatively soft bioabsorbable material, and the surfaces of the drive recesses can be sheared, or stripped, by the drivers. Additionally, single blade and hexagonal drivers tend to force the walls of the drive recesses outwardly when rotated therein producing outward expansion, or "mushrooming" of bioabsorbable screws. Furthermore, some drive recesses extend the entire length of the bone screw, and these drive recesses require that a significant quantity of material be removed from the bone screw resulting in a reduction in strength of the bone screw and impairing the overall resistance of the screw to deformation and damage when being driven into bone. For similar reasons, bioabsorbable bone screws are generally limited to use in open surgery, as opposed to closed, or endoscopic, surgery, because it is advantageous in endoscopic techniques for the screws to be cannulated, i.e. include a central longitudinal bore, for insertion along a guide wire. Formation of the central bore involves removing additional quantities of material from the screw and, therefore, structurally weakens bioabsorbable screws.
Alternative drive recesses, such as those defining multiple, radially oriented prongs for receiving similarly configured, multi-pronged drivers have been proposed for metallic screws, and illustrative drive arrangements are shown in U.S. Pat. Nos. 4,084,478 to Simmons; 3,872,904 to Barlow; 3,658,105 to Burt et al; 3,575,080 to Henney; 3,122,963 to Borgeson; 2,445,978 to Stellin; 2,445,525 to Gulden and 2,397,216 to Stellin. These drive recesses are formed in enlarged heads on metallic industrial screws, and typically taper longitudinally to a narrow end for engaging a similarly tapered driver. Multi-pronged drive recesses designed for metallic screws generally cannot be employed successfully in bioabsorbable bone screws because the forces applied by compatible multi-pronged drivers to such drive recesses include outwardly directed force components that cause outward expansion, or "mushrooming", in bioabsorbable bone screws. Furthermore, the walls defining multi-pronged drive recesses are typically configured to permit outward expansion of the screw material separating the radial prongs of the drive recess when the associated driver imposes force on the walls. Although this configuration is acceptable for metallic screws, it further promotes "mushrooming" in bioabsorbable bone screws due to the inherent relative softness of bioabsorbable materials. Conventional multi-pronged drivers also produce shear on the walls of corresponding drive recesses; and, when utilized in bioabsorbable bone screws, the walls can be sheared off, or stripped, by the drivers. Furthermore, many conventional multi-pronged drive recesses have only a small quantity of screw material separating the radial prongs of the drive recesses, and bioabsorbable bone screws having these types of drive recesses would be particularly vulnerable to shear deformation and could not withstand high drive forces. Additionally, the longitudinal taper in conventional multi-pronged drive recesses results in high concentrations of drive forces being applied by the drivers at the narrow end of the drive recesses where there is relatively less screw material to resist deformation, and bioabsorbable screws having tapered drive recesses are likely to experience significant deformation when driven into bone.